|I have been looking at establishing some
form of ATV in Perth. Perth had a successful ATV repeater several years
ago, but the site was lost and the repeater has not been in full
AMATEUR TELEVISION "FORMAL GROUP" MEETING
Brian VK6TTV is calling a meeting of ATV enthusiasts to try to formalise the group and would like all interested amateurs to attend, the venue will be the Audio/Visual Centre at W.A.C.A.E. Person St. Churchlands. Room No.AVC 1. on the 26th of November please contact him on Phone 09-387-6797.
At the W.I.A. Meeting last Tuesday Brian told me that he is expecting about 15 people to attend this meeting and he is very encouraged by the response so far.
VK6RTV founding members
VK6RTV as received at Don's (VK6HK) QTH
More information at....http://www.vk6.net/vk6rtv.html
VK6RTV on air
On Thursday, 6th June 1996, Amateur Television Repeater VK6RTV commenced service on test from a site at Greenmount. The VK6RTV license is held by the Perth ATV Group.
The repeater transmits on 444.25 Vision, 449.75 MHz sound, with an erp of about 50 Watts directed in an arc from about 225 degrees to 315 degrees from Greenmount.
Receive modes (selected by DTMF command on 145.5 MHz) are:
(1) 426.25 MHz Vision/431.75 Sound with AM Vision, FM sound.
(2) 1250.0 MHz Vision, FM Vision with 6.0 MHz FM sound subcarrier.
(3) 1242.25 MHz Vision/1247.75 Sound with AM Vision, FM sound.
The temporary antennas, both receive and transmit, are at present only low gain, so unless a good path is available to Greenmount, retransmitted pictures tend to be noisy. The repeater test pattern however has produced excellent picture quality in the target area, providing a purpose built 444 Mhz antenna and preamp are used. Many domestic TV and VCR tuners will tune down to the 443-450 MHz channel in use for the repeater output. Don't expect much from the existing domestic TV antenna and "barefoot" receiver, though!
Transmission of DTMF code #1# on 145.5 MHz will activate the test pattern, which alternates with a menu of other commands for 10 minutes, unless there is a vision signal on the selected input.
The repeater will be available with test pattern and tone from 1000-1130 on Sunday.
Reports on this experimental on-going project would be of interest!
73...Don VK6HK for Perth ATV Group
VK6RTV closed down
The number of amateurs active on VK6RTV were only about 5.
Tony Howse VK6XP (SK) was the most active through VK6RTV, sending taped material of various interest.
The production of ATV programs was an issue, as it is time consuming and difficult. However today with cheap quality colour cameras, digital recording and editing, the possibilities are much greater.
With the loss of the 425 to 432 segment the 436 input could no longer be used, but the repeater still functioned with the 1.2 GHz input until 1998, when the site was lost.
A thank you to Cyril VK6CN (SK) who provided his QTH in Greenmount for the ATV repeater.
Noal.... now VK6FNAJ and Don VK6HK
VK6RTV on air
Another view of VK6RTV antennas
The existing ATV repeater could be re-activated, but in my opinion the technology has changed so much that a simpler option may be available.
When thinking ATV, this does not always mean an ATV repeater. The task is a big one to establish an ATV repeater, and this effort alone can stop development, as it requires the dedication of one or more amateurs to see it through. What may encourage operation with ATV, is to first establish an ATV transmission, that amateurs can use to receive an ATV signal and develop their equipment and expertise.
Not all television transmissions are the same. There is....
This has to be remembered when considering the signal strength needed to receive a television signal. The actual signal strength in round figures has to be about 25dB stronger than an AM, FM or SSB audio transmission.
A 1uV signal on AM, FM or SSB is easily copied but for television you need about 20 to 50uV for a viewable picture.
The reason for this is the wideband reception. The wider the bandwidth the more noise in that bandwidth.
For every 10 times the bandwidth the noise level is 10dB greater....
A voice receiver with 10KHz bandwidth to a TV receiver with 6MHz bandwidth can be shown as...
10KHz 0dB Voice receiver.
6MHz +26dB More noise TV receiver.
Easy WayI have always been of the opinion that the best way to go is the easiest way. If you can achieve the easiest way then you can move onto more complex systems. If you fail at the easiest way, then less has been lost in terms of time effort and money.
Of the 3 methods of modulation, frequency modulation is the easiest way to go, and the reasons are, equipment.
When deciding what mode and band to use, the decision has to be based on what is easier for the user. If what ever setup is decided, makes it difficult for the amateur to get on air, then numbers involved in ATV will be limited. Transmission is the most difficult part for the amateur.
After much thought and discussion with other amateurs, I have come to the conclusion that the system used by VK6RTV has much to offer. 1.2GHz input and 444MHz output. The 444MHz band has the best propagation, and if amateurs can't see ATV they won't spend the time and effort to give it a go.
Then and Now
At lot has changed in the last 10 or so years, and that is equipment. The dedicated ATVer way back had to build or modify a lot of his equipment, along with finding it first. This tended to make ATV a specialist activity with amateurs, and hence limited numbers.
Today the range of equipment is amazing, and in terms of ATV, FM vision and sound modulation is what most of the transmitters and receivers use.
As said the range of equipment to experiment in ATV is only limited by your imagination.
Transmitters for many amateur bands are small, cheap and easy to obtain. This equipment is not specifically designed for amateurs, but a range of mass market applications such as security, model plane applications, and the hobbiest in general is there in abundance.
When talking equipment we are not just talking transmitters, but receivers and a variety of video signals, such as cameras, digital storage, remote control security cameras, the list is amazing.
In thinking of ATV, one of the first questions is what frequency? ATV transmissions are allowed on all bands above 430 MHz. However only FM modulation is allowed above 1.2GHz.
1.2GHz for the user
The important consideration when choosing a frequency is equipment and propagation. There is no ready to go mass market 1.2GHz equipment. The 1.2GHz mass market equipment is "available" but the frequency used is just below our 1.2GHz amateur band. Modification to frequency may be possible but adds difficulty and it is best avoided, so as to make ATV as easy as possible.
There is however ready to go 1.2GHz ATV equipment available, specifically for our amateur band from Minikits....Both TX and RX.
This is a crowded band with LIDPs occupying much of our amateur band. However there is a lot of transmitter and receiver equipment available, including digital transmitters, using the same digital encoding as the domestic free to air transmissions.
This band is similar to 2.4GHz in terms of equipment (except digital) but may have less problems with LIPDs as there is at least one ATV frequency outside the LIPD band.
When it comes to choosing one of the 3 bands (1.2, 2.4, & 5.8), propagation has to be considered. If we want as many amateurs as possible to become involved in ATV, then no point in making it hard, in terms of the frequency band chosen. If you can't see it you will not enjoy ATV.
The higher in frequency, the more difficult the propagation. 2.4GHz and in particular 5.8GHz are strictly line of sight (LOS). If you can't "see" the transmitter antenna, then you are unlikely to receive a signal. In particular vegetation attenuates these high frequencies a great deal.
The 1.2GHz band, is in my opinion, the best band to use for ATV by the amateur. The equipment is available, specific for amateurs.
Finding a site for an ATV transmitter and in the longer term an ATV repeater is perhaps the most difficult issue. The original ATV repeater ceased operation when the site it operated from was no longer available.
There are only 2 good sites on the scarp to the East of Perth that we as amateurs have access to now, and that is Tic Hill and Roleystone.
Tic Hill has the room but at the moment not enough power, and security is an issue.
Roleystone has limitation on the tower.